Method for Producing Aluminum Alloy Extrusion

ABSTRACT

A method for producing an aluminum alloy extrusion includes: conducting extrusion processing using a casted billet of an aluminum alloy containing 6.0 to 7.0% by mass of Zn, 1.5 to 2.0% by mass of Mg, 0.20 to 1.50% by mass of Cu, 0.10 to 0.25% by mass of Zr, 0.005 to 0.05% by mass of Ti, 0.15 to 0.35% by mass of Mn, 0.25% by mass or less of Sr, content of Mn and Zr and Sr being 0.10 to 0.50% by mass, with the balance being Al and inevitable impurities to obtain an aluminum alloy extrusion; cooling the extrusion to 100° C. or less at a cooling rate of 50 to 750° C./min immediately after the extrusion processing; and then conducting an aging treatment which is performed in one-stage or two-stage and a heat treatment which is performed at higher temperature for a shorter time than the aging treatment.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation of International Patent Application No. PCT/JP2021/001827, having an international filing date of Jan. 20, 2021, which designated the United States, the entirety of which is incorporated herein by reference. Japanese Patent Application No.2020-016671 filed on Feb. 4, 2020 is also incorporated herein by reference in its entirety.

BACKGROUND

The present disclosure relates to a method for producing an extrusion using an aluminum alloy, and relates more particularly to a method for producing an aluminum alloy extrusion that has not only high strength but also excellent moldability and corrosion resistance.

In the fields of automobiles, various industrial machines and the like, there is a demand for further reduction in weight and size, and as one of means for achieving the demand, production of a structural member from an aluminum alloy member having high strength is under examination.

As high-strength aluminum alloys, Al—Mg—Si-based (6000 series) alloys and Al—Zn—Mg-based (7000 series) alloys are known.

An object of a 6000 series alloy is to increase the strength by Mg₂Si precipitation hardening, but when the added amounts of Mg and Si are large, the 6000 series alloy has a technical problem that the extrudability is markedly deteriorated.

A 7000 series alloy is a natural aging type alloy, and has a characteristic that addition of Zn less affects the extrudability than that of Mg and Si, but has a technical problem that heat treatment time is long to obtain high strength by artificial aging treatment.

JP-A-2002-235158 discloses a method for producing a high-strength aluminum alloy extrusion using baking temperature at the time of painting, the alloy is however a 6000 series alloy, and adequately high strength is not obtained.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B illustrate the compositions of aluminum alloys used for evaluations (Examples);

FIG. 2 illustrates the compositions of aluminum alloys used for evaluations (Comparative Examples);

FIGS. 3A and 3B illustrate production conditions for casted billets and extrusions used for evaluations (Examples);

FIG. 4 illustrates production conditions for casted billets and extrusions used for evaluations (Comparative Examples);

FIGS. 5A and 5B illustrate results of the evaluations of extrusions (Examples); and

FIG. 6 illustrates results of the evaluations of extrusions (Comparative Examples).

DESCRIPTION OF EXEMPLARY EMBODIMENTS

The following disclosure provides many different embodiments, or examples, for implementing different features of the provided subject matter. These are, of course, merely examples and are not intended to be limiting. In addition, the disclosure may repeat reference numerals and/or letters in the various examples. This repetition is for the purpose of simplicity and clarity and does not in itself dictate a relationship between the various embodiments and/or configurations discussed. Further, when a first element is described as being “connected” or “coupled” to a second element, such description includes embodiments in which the first and second elements are directly connected or coupled to each other, and also includes embodiments in which the first and second elements are indirectly connected or coupled to each other with one or more other intervening elements in between. Further, when the first element is described as “moving” relative to the second element, such description includes embodiments in which at least one of the first element and the second element moves relative to the other.

An object of the present disclosure is to provide a method for producing a high-strength aluminum alloy extrusion that is excellent in corrosion resistance and moldability, and has good hardenability and high productivity.

In accordance with one of some embodiments, a method for producing an aluminum alloy extrusion includes: conducting extrusion processing using a casted billet of an aluminum alloy containing 6.0 to 7.0% by mass of Zn, 1.5 to 2.0% by mass of Mg, 0.20 to 1.50% by mass of Cu, 0.10 to 0.25% by mass of Zr, 0.005 to 0.05% by mass of Ti, 0.15 to 0.35% by mass of Mn, 0.25% by mass or less of Sr, content of Mn and Zr and Sr being 0.10 to 0.50% by mass, with the balance being Al and inevitable impurities to obtain an aluminum alloy extrusion; cooling the extrusion to 100° C. or less at a cooling rate of 50 to 750° C./min immediately after the extrusion processing; and then conducting an aging treatment which is performed in one-stage or two-stage and a heat treatment which is performed at higher temperature for a shorter time than the aging treatment.

As a result, in some embodiments, when, in an Al—Zn—Mg—Cu-based alloy, the extrusion is air-cooled immediately after the extrusion processing, the recrystallization depth in the surface of the extrusion can be suppressed, and hence, good hardenability and high strength can be obtained.

Now, reasons for setting the composition of the aluminum alloy will be described below.

Zn Component

Even when Zn is added in a comparatively large amount, high strength can be easily obtained with extrudability deterioration suppressed, but when excessively added, stress corrosion cracking resistance is deteriorated. Accordingly, the content of the Zn component is preferably in the range of 6.0 to 7.0%, and it is noted that “%” used means “% by mass”.

Mg Component

A Mg component is the most effective component for increasing the strength of an extrusion, but easily deteriorates the extrudability, and hence the extrusion is easily cracked in plastic working such as bending.

Therefore, the content of the Mg component is set to the range of 1.5 to 2.0%.

If the Mg component is in this range, the deterioration of the extrudability is suppressed, and high strength can be obtained.

Cu Component

A Cu component can increase the strength by a solid solution effect in a metal texture, but when added in a large amount, the Cu component tends to cause the deterioration of the extrudability and the moldability, and the general corrosion resistance is deteriorated.

Therefore, the content of the Cu component is set to the range of 0.20 to 1.50%, and preferably the range of 0.20 to 0.30%.

Mn, Cr, and Zr Components

Mn, Cr and Zr components are all transition elements, and have an effect of suppressing recrystallization that easily occurs in a surface of the extrusion in extrusion processing to reduce the depth of a recrystallized layer in the surface.

When the amounts of these components are large, the hardening sensitivity however becomes high in cooling (press end hardening) performed immediately after the extrusion processing.

Among those, the Cr component has a great influence thereon. The Cr component is not therefore incorporated, or if the Cr component is incorporated, the content thereof is suppressed to 0.05% or less, preferably 0.01% or less.

The Mn component has less strong hardening sensitivity than the Cr component, and the content thereof is satisfactorily in the range of 0.15 to 0.35% for adopting air cooling using a fan for press end hardening.

In the present disclosure, the Zr component is added to suppress the depth of this recrystallized layer, but the amount of Zr dissolvable in an aluminum melt is limited, and hence the content of Zr is in the range of 0.10 to 0.25%.

Sr and Ti Components

A Sr component can suppress coarsening of a crystal grain in textures of a casted billet, and as a result, has an effect of suppressing the depth of a recrystallized layer that is easily generated in a surface of the extrusion in the extrusion processing.

Meanwhile, when the amount of the Sr component added is large, a coarse crystallized product containing Sr as nuclei is easily generated.

When Sr is added, the content is therefore preferably suppressed to 0.25% or less. To attain both the strength and the suppression of a recrystallized layer, the total amount of [Mn+Zr+Sr] is preferably in the range of 0.10 to 0.50%.

If Cr is contained, the total amount of [Mn+Zr+Sr+Cr] is preferably in the range of 0.10 to 0.50%.

A Ti component is effective for the micronization of crystal grains in casting a billet, and the content of Ti is added is in the range of 0.005 to 0.05%.

Other Components

Examples of impurities easily mixed in casting a billet of an aluminum alloy include Fe and Si.

Since high contents of these components deteriorate the strength and bending workability, the content of Fe is preferably suppressed to 0.2% or less and the content of Si is preferably suppressed to 0.1% or less. In the present disclosure, the billet of an aluminum alloy as mentioned above is casted, and is air-cooled using a fan at a cooling rate of 50 to 750° C./min, preferably a cooling rate of 50 to 500° C./min as press end hardening immediately after extrusion processing.

When the cooling rate is over 750° C./min, a difference in cooling between different portions of the extrusion is made to easily cause strain.

Besides, a cooling apparatus used for water-cooling unavoidably has a large scale.

The extrusion is cooled near a normal temperature of 100° C. or less.

The casted billet to be used in the present disclosure is preferably subjected to homogenizing treatment at 480 to 520° C. subsequently to the casting and then cooled at a cooling rate of 50° C./hr or more so that the average crystal grain size is 250 μm or less.

The aluminum alloy extrusion according to the present disclosure is also characterized by the chemical composition of the alloy but the most characterized in that high strength can be obtained by heat treatment for a short time.

As mentioned above, the extrusion is air-cooled using the fan immediately after the extruding and then subjected to a heat treatment to obtain an extrusion that is excellent in stress corrosion cracking resistance (SCC resistance) and has high strength.

The extrusion was generally subjected to two-stage aging treatment which includes a first-stage aging treatment at relatively low temperature for generating primary crystals and a second-stage aging treatment at relatively high temperature for growing the primary crystals in the past, but this lengthens heat treatment time and causes productivity deterioration.

Therefore, the present disclosure can shorten a total time of heat treatment as compared with in the past by subjecting the extrusion to a heat treatment at higher temperature for a shorter time than the first-stage and the second-stage aging treatment after the aging treatment.

For example, the first-stage temperature is set to 90 to 180° C., preferably 110 to 150° C., and the second-stage temperature is set to a temperature higher than that of the first stage.

In the present disclosure, a subsequent heat treatment temperature is set to a temperature higher than that of the aging treatment.

For example, the extrusion is subjected to a heat treatment at a higher temperature (in the range of 140 to 250° C.) for a shorter time (for example, 0.1 to 6 hours) than the first-stage and second-stage aging treatment.

High strength, namely a tensile strength of 460 MPa or more and a proof stress of 430 MPa or more, can be obtained by such a heat treatment step.

In the present disclosure, the second-stage aging treatment may be omitted, and the above-mentioned heat treatment is performed after one-stage aging treatment in the case.

The extrusion of the aluminum alloy according to the present disclosure may be subjected to plastic working such as bending by press molding, bender bending or the like and then subjected to an artificial aging treatment or the like.

The casted billet of the aluminum alloy to be used for the present disclosure is continuously casted cylindrically with the chemical composition of the aluminum alloy in a melt adjusted to the aforementioned range. The setting of the casting rate to 50 mm/min or more and the cooling at a rate of 50° C./hr or more after the homogenizing treatment as mentioned above however enables the reduction of the average crystal grain size in the billet texture to 250 μm or less and the reduction of the depth of a surface recrystallized layer in the extrusion processing.

In the extrusion produced by the method for producing a high-strength aluminum alloy extrusion of the present disclosure, the phenomenon of cracks hardly occurring, namely what is called “stickiness”, is caused, and as a result, the stress corrosion cracking resistance is improved.

Besides, press end hardening may be performed by cooling means such as air-cooling using a fan, strain or deformation is difficult to occur in the extrusion, heat treatment time for an artificial aging treatment can be shortened, and the productivity is thus improved.

Exemplary embodiments are described below. Note that the following exemplary embodiments do not in any way limit the scope of the content defined by the claims laid out herein. Note also that all of the elements described in the present embodiment should not necessarily be taken as essential elements.

Aluminum alloy molten having compositions illustrated in FIGS. 1A, 1B and 2 were prepared to cast cylindrical billets.

The casting rates are illustrated in FIGS. 3A. 3B and 4. The billets were subjected to homogenizing treatment at HOMO temperatures (° C.), respectively.

The HOMO temperatures are preferably in the range of 480 to 520° C.

The extrusions were cooled at cooling rates illustrated in FIGS. 3A. 3B and 4, respectively.

“Billet Crystal Grain Size” illustrated in FIGS. 3A. 3B and 4 refers to an average crystal grain size in the texture of the casted billet.

The billet crystal grain size is preferably 250 μm or less in terms of the average grain size.

Each billet preheated to “BLT Temperature” illustrated in FIGS. 3A. 3B and 4 was charged in a container of an extruder for performing the extrusion processing.

Immediately after the extrusion processing, the extrusion was air-cooled using a fan at “Cooling Rate (° C./min)” in FIGS. 3A. 3B and 4 down to at least 100° C. or less.

The cooling rate is preferably in the range of 50 to 750° C./min.

The extrusion was bent into the arched shape on the assumption of the product shape of a bumper reinforcement, a door beam or the like if needed in the case of a vehicle component.

For example, the bending is performed at a curvature of 500 to 3000 mm.

Thereafter, a heat treatment was performed under “Heat Treatment Conditions” illustrated in FIGS. 5A and 5B and 6.

In the heat treatment conditions in FIGS. 5A and 5B and 6, “First Stage” refers to a heat treatment temperature and a heat treatment time at the first stage, “Second Stage” refers to a heat treatment temperature and a heat treatment time at the second stage, and “BH Temperature” and “BH Time” refer to high temperature heating conditions (heat treatment).

The evaluation results are illustrated in FIGS. 5A and 5B and 6.

The values of T5 refer to tensile strength (MPa), 0.2% proof stress (MPa), and elongation (%) after artificial aging treatment.

Target values according to the present disclosure are illustrated in FIGS. 5A and 5B and 6.

A JIS No. 5 test piece was prepared from each extrusion in accordance with JIS-Z2241, and these mechanical properties were measured using a tensile testing machine compliant with JIS specification.

The crystal grain size of each billet and the surface recrystallization depth of each extrusion were measured by performing image processing through observation using an optical microscope after subjecting the cross-section to mirror finishing, and then to a prescribed etching treatment.

In FIGS. 5A and 5B and 6, “SCC Property” refers to the results of a test for stress corrosion cracking resistance.

Each test piece was subjected to 720 cycles under a stress corresponding to 80% of the proof stress with the following conditions defined as one cycle, and a test piece in which no cracks occurred was estimated to achieve a target.

One Cycle:

The test piece was immersed in a 3.5% NaCl aqueous solution at 25° C. for 10 minutes, then allowed to stand for 50 minutes at a temperature of 25° C. and a humidity of 40%, and then naturally dried.

Discussion on Evaluation Results

Examples 1 to 45 have achieved all the quality targets.

Especially Examples 19, 22, and 25 and Examples 34 to 38 are also excellent in elongation or SCC resistance while achieving a tensile strength of 460 MPa or more and a proof stress of 430 MPa or more even though the total heat treatment time (“Whole Time”) is suppressed to 10 hours or less.

Meanwhile, Comparative Examples 8 to 11 had small amounts of Mn and Mg added, and did not achieve the tensile strength and the proof stress.

Since Comparative Example 12 had a large amount of Mg and also a large amount of Cu, the SCC resistance did not achieve the target.

The first-stage heat treatment times of Comparative Examples 1 to 4 are set to as long time as in the past, but the tensile strengths do not reach the levels of Examples 1 to 3.

In accordance with the production method according to the present disclosure, the extrusion has high strength, excellent corrosion resistance, and good moldability, and therefore is applicable to structural members for various vehicles or industrial machines.

Although only some embodiments of the present disclosure have been described in detail above, those skilled in the art will readily appreciate that many modifications are possible in the embodiments without materially departing from the novel teachings and advantages of this disclosure. Accordingly, all such modifications are intended to be included within scope of this disclosure. 

What is claimed is:
 1. A method for producing an aluminum alloy extrusion includes: conducting extrusion processing using a casted billet of an aluminum alloy containing 6.0 to 7.0% by mass of Zn, 1.5 to 2.0% by mass of Mg, 0.20 to 1.50% by mass of Cu, 0.10 to 0.25% by mass of Zr, 0.005 to 0.05% by mass of Ti, 0.15 to 0.35% by mass of Mn, 0.25% by mass or less of Sr, content of Mn and Zr and Sr being 0.10 to 0.50% by mass, with the balance being Al and inevitable impurities to obtain an aluminum alloy extrusion; cooling the extrusion to 100° C. or less at a cooling rate of 50 to 750° C./min immediately after the extrusion processing; and then conducting an aging treatment which is performed in one-stage or two-stage and a heat treatment which is performed at higher temperature for a shorter time than the aging treatment.
 2. The method as defined in claim 1, wherein the casted billet is subjected to homogenizing treatment at 480 to 520° C. subsequently to casting and then cooled at a cooling rate of 50° C./hr or more so that an average crystal grain size is 250 μm or less.
 3. The method as defined in claim 2, wherein the aluminum alloy extrusion has a tensile strength of 460 MPa or more and a proof stress of 430 MPa or more. 